1. Field of the Invention
The present invention relates generally to a systems and processes for heat treatment of glass and more specifically to heat treatment of glass in a tempering system.
2. Description of Related Art
In the heat treatment of glass, especially in a tempering system or tempering furnace, there are conventionally radiant heaters to heat the glass. The heat is injected at least in large part via heat radiation.
In particular, in so-called low-E glass (i.e., glass with low emission, conventionally by a reflective coating) a problem exists where heat injection, which is much less compared to normal glass, is possible by heat radiation. Alternatively or in addition, therefore there is heat transfer by convection (a warm or hot air flow being directed at the glass or routed past it) to improve heat exchange.
A similar problem, especially relevant again with low-E glass, resides in “key formation” as the flat glass enters such a system. The still cold incoming glass rolls on ceramic rollers of a conveyor means, where the rollers are of the temperature of the heating chamber (a temperature of roughly 700° C.). The heat transfer from the rollers to the bottom of the glass is very intensive, but the heat conduction of the glass, as a result of its low temperature, is still low. The heat radiation from the top is less effective, especially for low-E glass, so that the glass plate in the heating chamber has the tendency to arch in the manner of a key. This is especially pronounced in low-E glass in which the reflective coating is not on the top side that comes into contact with the rollers, in order to avoid damage to the coating.
U.S. Pat. No. 4,059,426, which comprises the starting point of this invention, discloses a system for heat treatment of glass. According to one version, compressed air is blown into the heating chamber by means of a propellant air fan, also called an air flow amplifier, which is located vertically above the glass. The compressed air is fed tangentially into the propellant air fan which, based on the Coanda effect, conveys the warm air located in the heating chamber through a diffuser which widens conically.
The entire volumetric flow which flows through the propellant air fan is greater by a volumetric factor of 10 to 20 than the volumetric flow of supplied fan air. In order to achieve the required high circulation of air in the heating chamber, a comparatively large volumetric flow of fan air must be supplied at the indicated volumetric factor. In order to prevent excess cooling of the air in the heating chamber by the supplied fan air, the fan air is preheated.
Published European Patent Application EP 0 058 529 A1 discloses a similar system for heat treatment of glass. In contrast to the aforementioned propellant air fans, open nozzles are arranged in rows transverse to the conveyor direction of the glass, in order to direct fan air from top and bottom onto the glass surface. The glass is conveyed in the heating chamber on rollers.
The systems for heat treatment, described in published International Patent Application WO 95/04708 A1 and in U.S. Pat. No. 5,368,624, likewise have propellant air fans for controlled delivery of air. These propellant air fans are in any case intended solely for cooling of the glass which is to be treated.
Another known heat treatment system, as described in published International Patent Application WO 00/37371 A1, shows a heating means for radiant heating underneath the glass which is to be treated and several hot air nozzles above the glass which is to be treated for heat transfer by convection. Here, the hot air nozzles are used solely to deliver fan air. The disadvantage in this concept is the danger of spot heating of the glass to be treated.